Despite the importance of CpG methylation and chromatin modifications in normal development and in disease, little is known about how methylation and chromatin patterns are established in mammals, and, specifically, how sequences are marked for inactivation. The investigator discovered a locus, Ssm1 (strain-specific modifier), on chromosome 4 of the mouse that has a major effect on the methylation and chromatin modification of a complex transgene, HRD, and certain derivatives. Methylation and inactivation occurs in a subset of mouse strains, including C57BL/6 (B6), but not other strains, including DBA/2 (D2). Methylation is dominant;(B6xD2) F1 mice methylate the target. Ssm1 is one of the very few mammalian loci shown to affect the methylation/chromatin status of specific target sequences. Determining how Ssm1 acts will significantly advance our understanding of epigenetic mechanisms in mammalian development and health. Ssm1 may be a member of a regulatory system to mark sequences for inactivation, involving complex interactions of allelic and non-allelic modifiers that are encoded by different as well as overlapping genes in different mouse strains and, by inference, different human individuals. The investigator has mapped Ssm1 to a narrow genomic interval on mouse chromosome 4 that is syntenic with human chromosome 1p36, a region involved in tumorigenesis and neurological defects. The defined genomic interval contains several potential Ssm1 candidates, all are KRAB-zinc finger genes that may encode proteins that bind their DNA target via the zinc finger, and recruit repressive proteins via the KRAB domain. She proposes to eliminate the B6 allele of Ssm1 candidates by gene replacement with a neo gene to identify Ssm1 as that gene whose elimination prevents inactivation of the HRD target. These experiments should elucidate how DNA methylation and chromatin modifications are regulated during early embryonic development and throughout life. Ssm1 orthologs in man may have clinical relevance, e.g., for epigenetic events in tumorigenesis and tumor progression, as well as for modification of MeCp2 function in neurological disorders.